Films are used industrially for antireflection coatings, front surface mirrors, interference filters, sunglasses, or decorative coatings on plastic and textiles; in the manufacture of cathode ray tubes; in electronic circuits; in the production of magnetic films for computer information storage; and in the production of photovoltaic cells. One way to produce thin films is using a technique known as vacuum evaporation.
Although commonly referred to as a single process, the deposition of thin films by vacuum evaporation consists of several distinguishable steps:
1. Producing vapor;
2. Conveying vapor from the source to the substrate; and
3. Condensing the vapor on the substrates.
Thin film photovoltaic cells can be produced by elemental coevaporation. To obtain the desired film composition, the evaporation rates of the materials must be accurately controlled. As the scale of the production process increases and the volume of the source for the evaporated material increases, it becomes increasingly more difficult to control the evaporation rates accurately. The larger volumes of elemental evaporants and size of heating equipment contribute to the difficulty in controlling the evaporation rates precisely.
The energy needed for evaporation can be provided by resistance heating, electrical induction, lasers, electron bombardment, or a combination of these means. In one prior art system, a tantalum foil heater provides indirect radiant energy to a vessel that contains the material to be evaporated. An insulating gap separates the heater from the vessel. The ends of the tantalum heater are attached to rigid electrical contacts. Adjustments of the power control for the heater are common to control the energy produced by the heater. One drawback of this system is its inability to quickly translate adjustments in the power control to changes in the temperature of the material being evaporated. Also, since the ends of the tantalum heater are attached to rigid electrical contacts, during thermal cycling, the heater can buckle and crack due to stresses induced by thermal expansion and compression. Cracking of the heater varies the the resistance of the heater which results in current fluctuation and an inability to predictably control the amount of energy produced by the heater.
To obtain accurate control of the evaporation rate from a resistance heat source, it is desirable to overcome such problems. A resistance heat source should enable the user to quickly change the temperature of the material being evaporated by controlling the input power. To provide a heat source that provides a relatively constant and predictable current, cracking of the heater should be avoided.